Several lines of evidence indicate that the Moloney strain of murine sarcoma virus (MoMuSV) expresses its src gene as a separate gene product. We and others have been unable to produce an antibody specific for the Moloney src gene product. We have taken another approach, that of translating short poly(A) fragments from genomic RNA preparations of Mo-MuSV-124. Our findings indicate that the 5' half of the genome codes for a defective gag gene product (P63gag), whereas the 3' half translates into a P38 and P23. P23 has been thoroughly characterized structurally and found not to be related to any product of the replication genes. P38 and P23 share peptide sequences and appear to be derived from that portion of the genome that contains the src gene. A temperature sensitive (ts) mutant (ts110) of Mo-MuSV with a ts defect in a function required to maintain the transformed state was investigated. Virus infected cells express two viral proteins at temperatures permissive for the transformed state, termed P85 and P58. At temperatures in which the cells revert to normal, only P58 is expressed. Peptide mapping studies have shown that P85 contains sequences of viral structural proteins p15, p12 and part of p30 as well as sequences of the candidate Moloney src gene product termed P23. Thus, P85 is a candidate gag-src fusion protein. The synthesis of P85 preceeds several changes characteristic of malignant transformation as ts110 MuSV infected cells are shifted to the permissive temperature. Thus, P85 synthesis was detected 2 hr after temperature shift, whereas morphological transformation began to occur after 5 hr, increased hexose uptake was observed at 8-12 hr, alterations in the cytoplasmic microtubule complex were detected 5 hr after shift, but F-actin cables did not begin to change until 24 hr. Enucleation of cells at nonpermissive temperature and shifting cytoplasts to permissive temperature did not result in synthesis of detectable P85 or in any alteration of the cytoplast morphology or microtubules. Other studies have identified a MuSV-induced P55 that is unrelated to any viral coded protein. A Rauscher murine leukemia virus associated DNase was partially purified. A 40,000 dalton viral coded polypeptide derived from Pr200gag-pol co-purifies with the enzyme.